The present invention relates to fuel cells that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to improved mass transport of water and oxidant within the fuel cell.
Fuel cells are well known and are commonly used to produce electrical energy from hydrogen containing reducing fluid and oxygen containing oxidant reactant streams to power electrical apparatus such as motors, and transportation vehicles, etc. In operation of a fuel cell, the electrochemical reaction of the reducing fluid and oxidant at anode and cathode catalyst surfaces of the electrolyte produces an electrical current and also produces product water at the cathode catalyst. In fuel cells of the prior art, it is well known that as a fuel cell reaches its maximum rated power output and current density increases, increased production of product water at the cathode catalyst may flood the cathode catalyst and thereby impede flow of the oxidant reactant stream by the cathode catalyst. As flow of the oxidant is impeded by flooding of the cathode catalyst, production of electrical current is also limited. Therefore, inadequate mass transport of oxidant to and product water away from the cathode catalyst severely limits fuel cell performance at high fuel cell current densities.
Known efforts to enhance mass transport of product water include utilization of hydrophilic components that define a cathode flow field adjacent the cathode catalyst or cathode electrode. For example, it is common to add hydrophilic rendering compounds to porous carbon materials making up support layers adjacent to the cathode catalyst. The pores of such carbon support materials define a cathode flow field for directing the oxidant to flow adjacent the cathode catalyst and for directing the product water to flow away from the cathode catalyst. Hydrophilic carbon support layers are shown in U.S. Pat. No. 5,998,058, which issued on Dec. 7, 1999, and that is owned by the owner of all rights in the present invention, and in a U.S. Patent Application published on Jun. 13, 2002 under Ser. No. U.S. 2002/0071978 A1, that is also owned by the owner of all rights in the present invention. It is also known to utilize a porous water transport plate in fluid communication with the cathode flow field, wherein a coolant stream passing through the plate flows at a pressure that is less than a pressure of the oxidant flowing through the cathode flow field so that product water will readily move into the porous water transport plate, such as disclosed in U.S. Pat. No. 6,316,135 that issued on Nov. 13, 2001, and is also owned by the owner of all rights in the present invention. Such efforts, however are also known to be costly, and may include additional bulky components. Frequently addition of hydrophilic rendering compounds will present other difficulties, not just in cost, but for example in limiting electrical conductivity of the components.
A further effort at enhancing mass transport within a fuel cell is disclosed in a Japanese Patent Application No. 11-313,691, published on May 18, 2001, under Publication No. 2001-135338. It discloses utilization of a supply of preferably ethanol and injectors to inject some of the ethanol into the reactant streams passing through a fuel cell so that the ethanol decreases a contact angle formed by a surface of condensed water droplets, of for example product water, and a surface for directing the reactant gases through the fuel cell, in order to improve drainage of the condensed water out of the cell. While such an approach may enhance mass transport of fuel cell water, it necessitates use of one or more injectors, storage of ethanol or similar compounds with the fuel cell, and passage of those compounds with the reactant streams out of the fuel cell to present additional problems related to environmental hazards of fuel cell exhaust gases.
It is desirable, therefore, to develop a method of operating a fuel cell that efficiently enhances mass transport of fuel cell water and oxidant at high current densities of an operating fuel cell.
The invention is a method of using a temporary dilute surfactant water solution to enhance mass transport in a fuel cell that generates electrical current from hydrogen containing reducing fluid and oxygen containing oxidant reactant streams. The fuel cell includes a cathode catalyst and an anode catalyst on opposed surfaces of an electrolyte, a cathode flow field for directing the oxidant reactant stream to pass adjacent to the cathode electrode, and an anode flow field for directing the reducing fluid reactant stream to flow adjacent to the anode electrode. The method of using the temporary dilute surfactant water solution to enhance mass transport of the cell includes the steps of: a. directing a temporary dilute surfactant water solution to flow through at least one of the flow fields, wherein the temporary dilute surfactant water solution has a surface tension of not less than 50 dynes per square centimeter (hereafter xe2x80x9cdynes/cmxe2x80x9d); b. then, removing the temporary dilute surfactant water solution from the fuel cell; and, c. then directing the oxygen containing oxidant reactant stream to flow through the cathode flow field and directing the hydrogen containing reducing fluid stream to flow through the anode flow field.
During experiments by the inventors of the present invention to ascertain a value of temporary antifreeze surfactant water solutions of varying concentrations within a fuel cell, it was unexpectedly determined that an application of a dilute surfactant water solution flowing through flow fields of the cell for a temporary, short duration improved mass transport of the cell even after the dilute surfactant water solution stopped flowing through the cell and was removed from the cell. It is theorized that the dilute surfactant water solution having a surface tension of not less than 50 dyne/cm partially wets a porous catalyst support layer and/or the catalyst, wherein pores of the layer define the flow field through which the dilute surfactant solution flows. The wetting of the porous support layer and/or the catalyst may establish permanent water transfer paths that facilitate mass transport of product water from the cathode catalyst, thereby resulting in improved transport of oxygen to the cathode catalyst, all of which enhances fuel cell performance.
In preferred methods of using a temporary dilute surfactant water solution to enhance mass transport, the dilute surfactant water solution may be directed to flow through a porous water transport plate within a liquid coolant stream, wherein the porous water transport plate is in fluid communication with at least one of the flow fields of the cell, and preferably, with the cathode flow field. The temporary dilute surfactant water solution may also be flowed through the fuel cell for a pre-determined time prior to a break-in period, wherein generation of electrical current by the fuel cell for the first time has achieved 100% of a rated power. By the phrase xe2x80x9crated powerxe2x80x9d, it is meant that both current and voltage from the fuel cell meet design specifications of the fuel cell.
In further preferred methods, the dilute surfactant is selected from the group consisting of methanol, ethanol, propanol, isopropyl alcohol, acetone, acetic acid, acetaldehyde, and propylamine, or mixtures thereof. In a further preferred method, the temporary dilute surfactant water solution is about a 12.5% methanol water solution. In a further preferred method, the dilute surfactant is selected from the group consisting of surfactants having a solubility in water that is greater than 25 grams per 100 grams of water, having a surface tension not greater than 35 dynes/cm, and having a boiling point of not greater than 120xc2x0 C.
Accordingly, it is a general purpose of the present invention to provide a method of using a dilute surfactant solution to enhance mass transport in a fuel cell that overcomes deficiencies of the prior art.
It is a more specific purpose to provide a method of using a dilute surfactant solution to enhance mass transport in a fuel cell by facilitating removal of product water from the fuel cell and by facilitating transfer of an oxidant to a cathode catalyst of the fuel cell.
These and other purposes and advantages of the present method of using a dilute surfactant solution to enhance mass transport in a fuel cell will become more readily apparent when the following description is read in conjunction with the accompanying drawing.